Abstract. LEXEO Therapeutics, LLC, is an early stage biotechnology company focused on using in vivo gene therapy technologies to treat disorders of unmet medical need. LEXEO is developing an in vivo gene therapy strategy as a preventative therapy to protect individuals with aldehyde dehydrogenase 2 (ALDH2) deficiency from the high risk for esophageal cancer. As the next step in the development path, the goal of this Phase I STTR is to demonstrate that a single in- travenous administration of LEX06, a serotype rh.10 adeno-associated virus coding for human ALDH2 will suppress the chronic ethanol ingestion-associated development of cancer related DNA damage and adducts in the esophagus of ALDH2 deficient mice. ALDH2 deficiency, a com- mon hereditary disorder affecting 560 million people (8% of the world population), has a high preva- lence in people of East Asian background (35-45%). ALDH2 is a key enzyme for ethanol metabolism; mutations that reduce the oxidizing ability of the enzyme result in systemic accumulation of toxic acet- aldehyde. With ethanol ingestion, the dysfunction of ALDH2 is linked to the ?Asian flush syndrome,? with facial flushing, headache, nausea, dizziness and cardiac palpitations. Most importantly, affected individuals have a 7- to 12-fold increased risk of cancer of the oral cavity, pharynx, larynx, and esoph- agus. The combination of cigarette smoking and alcohol consumption with the ALDH2*2 genotype is associated with one of the highest known cancer risks (odds ratio 50:1) and a 25 yr earlier onset of esophageal carcinoma. At present, there is no preventative therapy for the increased risk for esopha- geal cancer in this population. The long-term expression and impact of the gene therapy will be as- sessed in 2 mouse models of ALDH2 deficiency chronically exposed to ethanol to induce acetalde- hyde accumulation, esophageal DNA damage and adducts. We hypothesize that LEX06 therapy will prevent the accumulation of cancer risk-associated DNA damage and adducts induced by chronic ethanol ingestion. The demonstration that LEX06 will prevent esophageal DNA damage and adduct accumulation in these mouse models will be sufficient preclinical efficacy data to initiate an STTR phase 2 product development to move toward an issued IND to carry out initial clinical stud- ies. We propose the following aim. Aim 1. To evaluate the hypothesis that LEX06 (AAVrh.10hALDH2) therapy will prevent ethanol-induced DNA damage and adduct formation in the esophagus in ALDH2-/- and ALDH2E487K+/+ mice. Serum levels of acetaldehyde will be moni- tored. Markers of DNA damage and acetaldehyde-induced DNA adducts will be assessed in esopha- geal tissues. Milestone. To demonstrate that at doses relevant to humans, LEX06 will reduce esoph- ageal DNA damage and adduct formation in both mouse models by >50%.